The present invention relates to a dual-feed antenna.
When a tank is deployed in a battlefield it is desirable for both the commander and the gunner to be able to view potential targets. A commander will survey the battlefield to identify and prioritise targets using a conventional infrared or optical range-finder sight. An antenna usually is aligned with (is a slave to) the inferred or optical range-finder sight. When the commander believes that he has identified a target, he will activate the antenna, and a communication system will confirm whether the target is a “friend” or “foe”, and/or to obtain range or other information. The location of the non-“friendly” targets is provided to a target management system which then aims the gun appropriately. Before firing the gun the gunner needs to use an antenna and communication system to verify his or her aim and/or to confirm that the target has been correctly identified and is not a “friendly” target.
If a completely separate antenna and communication system is provided for each of the commander and gunner this increases cost and weight. An alternative system may comprise a single antenna, which is shared by the commander and gunner. The antenna is used most of the time by the commander. The commander uses the antenna to survey the battlefield. Only when the gunner is taking aim and needs to confirm his or her aim and/or the target identity immediately prior to firing the gun will the antenna be moved to align with the gun bore-sight. When the antenna is aligned with the gun bore-sight, and during the time that the antenna is moving from the position chosen by the commander to be aligned with the gun bore-sight, the commander cannot access the communication system. It is desirable to minimise the time that the commander loses his or her access to the communication system, even though in modern systems this time is typically less than one second.
It is an object of the present invention to provide a communication system which allows two antenna feeds to be provided without duplicating the entire communication system.
According to the present invention there is provided an antenna comprising first and second electromagnetic radiation paths which join to form a common third electromagnetic radiation path, wherein the first path is rotatable with respect to the second path. In this way, two antenna feeds can be directed independently of one another, whilst combining the electromagnetic radiation paths from the antenna feeds then allows a common set of components to be used in signal analysis or generation.
Optionally, the first and second paths are independently rotatable about a common axis. This allows a simplified design. The first path may be rotatable in tandem with an infra red or optical sight and, optionally, the second path may be rotatable in tandem with a directional weapon. This would match the requirement for a tank in which a commander uses the infra red or optical sight and a gunner uses the directional weapon.
Preferably, the antenna comprises an electromagnetic radiation modifier operable to modify a property of radiation passing along at least one of the first or second paths. This is a convenient way of rendering radiation following one of the paths distinguishable from radiation following the other path. Conveniently, the modifier may be a polariser operable to change the polarity of radiation passing along at least one of the first or second paths. Advantageously, the antenna comprises first and second polarisers associated with the first and second paths respectively, the first and second polarisers being operable to produce substantially orthogonal linear polarisations of radiation passing along the respective paths. Devices are known, such as polarising filters, for allowing the passage of radiation which is linearly polarised in one direction whilst not allowing the passage of radiation linearly polarised in an orthogonal direction, and this is a convenient way of separating the radiation of the two paths.
Optionally, the antenna further comprises a switch operable selectively to allow radiation from the first and/or second path to pass along the third path. The switch may be sensitive to polarisation direction and/or frequency.
Preferably, the first path includes a first reflector for directing radiation to the third path and, optionally, the second path may include a second reflector for directing radiation to the third path. The first reflector may rotate with the first path or may be fixed in position relative to the first path. Similarly, the second reflector may rotate with the second path or may be fixed in position relative to the second path.
The second reflector may be a wire grid operable to reflect radiation from the second path to the third path and to transmit radiation from the first path to the third path. Conveniently, the wire grid is operable to transmit radiation from the second path when first incident to a third reflector that is operable to reflect radiation back towards the wire grid whereupon the radiation is reflected to the third path. Optionally, the third reflector rotates the polarisation of any instant radiation by 90° upon reflection.
Of course, a single antenna may allow receiving and transmission of radiation along the two paths.